Modulate pacing rate to increase the percentage of effective ventricular capture during atrial fibrillation

ABSTRACT

The present disclosure pertains to cardiac pacing methods and systems, and, more particularly, to cardiac resynchronization therapy (CRT). In particular, the present disclosure pertains to determining whether a patient is experiencing atrial fibrillation (AF). If the patient is experiencing AF, the efficacy of CRT is determined. A signal is sensed in response to a ventricular pacing stimulus. Through signal processing, a number of features are parsed from the signal and a determination is made as to whether the ventricular pacing stimulus evoked a response from the ventricle.

FIELD

The present disclosure pertains to cardiac pacing methods and systems,and, more particularly, to cardiac resynchronization therapy (CRT).

BACKGROUND

Cardiac resynchronization therapy devices operate by either deliveringpacing stimulus to both ventricles or to one ventricle with the desiredresult of a more or less simultaneous mechanical contraction andejection of blood from the ventricles. Ideally, each pacing pulsestimulus delivered to a ventricle evokes a response from the ventricle.Delivering electrical stimuli that causes the ventricle to respond iscommonly referred to as capturing a ventricle.

For a variety of reasons, cardiac pacing systems may not effectivelycapture a ventricle. For example, a patient experiencing atrialfibrillation (AF) may have an implantable cardioverter-defibrillator(ICD) that shows a very high percentage (e.g., greater than 90%) ofdelivered biventricular (BiV) paces compared to sensing of intrinsicactivations. However, BiV pacing may be ineffective if the paces aredelivered to myocardial tissue in a refractory state (i.e. subnormalexcitability of myocardial tissue following excitation), resulting inpseudo-fusion. Pseudo-fusion involves electrical activation ofventricles almost entirely through intrinsic electrical activity withminimal or no contribution from pacing of the ventricle(s).Pseudo-fusion can occur during AF, because AF results in irregularconduction of atrial impulses to the ventricles. In turn, irregularventricular activation rate increases the chance of inconsistent captureof the ventricles. Typically, delivery of substantially consistent BiVpacing during AF is achieved by overdriving the ventricular intrinsicrate. However, overdriving ventricular rate can lead to unfavorablecardiac mechanics and can exacerbate heart failure. Therefore, it isdesirable to increase the percent of effective BiV pacing during AFwithout significantly increasing the ventricular rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary system including an exemplaryimplantable medical device (IMD).

FIG. 2 is a diagram of the exemplary IMD of FIG. 1.

FIG. 3A is a block diagram of an exemplary IMD, e.g., the IMD of FIGS.1-2.

FIG. 3B is yet another block diagram of one embodiment of IMD (e.g. IPG)circuitry and associated leads employed in the system of FIG. 2 forproviding three sensing channels and corresponding pacing channels thatselectively functions in a ventricular pacing mode providing ventricularcapture verification.

FIG. 4 is a flowchart of an exemplary normal mode employed by animplantable medical device to modulate the biventricular (BiV) pacingrate to increase the percentage of effective BiV capture during atrialfibrillation.

FIG. 5 is a flowchart of an exemplary duty cycle mode employed by animplantable medical device to modulate the BiV pacing rate to increasethe percentage of effective BiV capture during atrial fibrillation.

FIG. 6 depicts an exemplary set of sequential beat signals that mayoccur during a normal mode of operation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

It will be apparent to a skilled artisan that elements or processes fromone embodiment may be used in combination with elements or processes ofthe other embodiments, and that the possible embodiments of suchmethods, devices, and systems using combinations of features set forthherein is not limited to the specific embodiments shown in the Figuresand/or described herein. Further, it will be recognized that theembodiments described herein may include many elements that are notnecessarily shown to scale.

One or more embodiments of the present disclosure are directed to animplantable medical device (IMD) configured to deliver bi-ventricularcardiac resynchronization therapy (CRT). The IMD (e.g. implantablecardioverter-defibrillator (ICD)) is configured to determine whetherpacing stimuli effectively captures a ventricle during atrialfibrillation (AF). Determining whether the ventricular pacing stimulusis capturing the paced ventricle occurs solely during atrialfibrillation. Depending upon whether effective capture is achievedduring AF, the heart rate target level may be incrementally adjusted.Exemplary incremental adjustments can be 1-5 beats per minute or 1percent (%), 2%, 3%, 4%, %5 or 10% of the HR target level. The IMD canuse a normal process mode of operation or a duty cycle mode of operationto adjust the target heart rate to increase effective capture during AF.Normal process mode generally entails the IMD continuously determiningwhether a pacing stimuli effectively captures a ventricle and adjustingthe target rate during AF. In contrast, the duty cycle mode conservespower by not continuously determining whether the pacing stimulieffectively captures the ventricle during AF. For example, the dutycycle mode may operate 10% of the time over a period of time (e.g.daily, weekly, etc.). Alternatively, the duty-cycle mode of operationcan be configured as a function of AF burden. For example, the dutycycle mode can operate a certain number of hours while AF is detected.

In yet another embodiment, a combination of the normal mode of operationand the duty cycle mode of operation can be employed. For example, anormal mode of operation can be used for the first N hours of AF per daywhere N is any number between 1 and 12 followed by a duty-cycle mode. Inanother embodiment, the feature as to determining effective capture isalways run in a duty-cycle mode.

In either case, a fixed or adjustable duty-cycle mode of X % can be thenemployed by the IMD where X may be some function of the number of hoursfor which the patient is in AF. For example, in one or more preferredembodiments, duty-cycle mode of X % means that the feature, related todetermining effective capture during AF and adjusting the target pacingrate in response, has the first 100 beats per hour are analyzed todetermine statistical data on effective capture while the remainingportion of the hour and, every subsequent hour thereafter, performs dutycycling in a pre-specified manner (e.g. 10 seconds operation out of each30 seconds).

In one or more other embodiments, duty-cycle mode of X % means that thefeature, related to determining effective capture during AF andadjusting the target pacing rate in response, is activated for the first(X/100)*60 minutes of an hour. For example, an adjustable duty-cycleoperation may run 100% for the first hour of AF, then duty-cycle maydecrease in fixed decrements of 4% for each extra hour. The duty-cyclemode would be 96% for the second hour, 92% for the third hour, . . . 18%in the 24th hour. For this particular example, the effective duty cyclefor 24 hours of AF would be 54%.

In still yet another embodiment, the duty cycle mode can operate apredetermined percentage of time. For example, the duty cycle mode canoperate 50% of a specified time period (e.g. 50% every minute etc.);therefore, the duty cycle mode would run every 0.5 minute.

In one or more preferred embodiments, upon activating the featurerelated to determining effective capture during AF and adjusting thetarget pacing rate in response, a non-duty cycle mode is configured torun for e.g. 30 seconds (“initialization period”). Thereafter, thisfeature would begin to operate in duty cycle mode.

The heart rate, measured at the end of the initialization period, isthen adjusted or augmented to a final designated pacing rate by 1 beatsper minute (BPM) if the heart rate is greater than 100 BPM, 3 BPM if theheart rate is greater than 80 BPM but less than 100 BPM, and 5 BPM ifthe heart rate is less than 80 BPM. The final designated pacing rate isthen applied during a hold period of 30 seconds. The hold period beginsafter the initialization period ends, and ends 30 seconds later. Duringthe hold period, the pacing rate can only be adjusted upward if acertain pre-specified number of sensed ventricular events are met out ofthe total number of ventricular events. For example, the pacing rate canonly be adjusted upward if 6 of 10 ventricular events in any period of10 ventricular events are sensed ventricular events, not pacedventricular events. If 6 of 10 ventricular events in any period of 10ventricular events are sensed, the heart rate can be increased by 2 BPMand the buffer of 10 beats is cleared. Battery power is conservedbecause determination of sensed versus paced events is very simplecompared to the determination of effective capture versus ineffectivecapture. At the end of the hold period, 10 beats are analyzed foreffective capture. Depending on how many of the 10 beats have effectivecapture, the pacing rate for the next 30 second hold period is adjustedup or down.

The present disclosure achieves effective capture during AF bydelivering pacing stimuli at sufficient energy and at the proper timing.Additionally, morphology analysis of paced electrograms is used todetermine effective capture, which provides beneficial results overknown capture management algorithms. While capture management algorithmsare able to artificially modify the timing (i.e., overdrive pace or usevery short SAV/PAV), the main focus of capture management algorithms ison sufficient energy delivery of a pacing stimulus. Capture managementalgorithms generally do not address proper timing and cannot be used toassess effective capture during normal device operation. Moreover,capture management algorithms generally do not optimally work during AFor fast heart rates.

Presented below is a description of the IMD hardware (FIGS. 1-3).Thereafter, a description is presented of one or more processes (FIGS.4-6) used for modulating the pacing rate in order to increase thepercentage of effective ventricular capture during AF.

FIG. 1 is a conceptual diagram illustrating an exemplary therapy system10 that may be used to deliver pacing therapy to a patient 14. Patient14 may, but not necessarily, be a human. The therapy system 10 mayinclude an implantable medical device 16 (IMD), which may be coupled toleads 18, 20, 22 and a programmer 24. The IMD 16 may be, e.g., animplantable pacemaker, cardioverter, and/or defibrillator, that provideselectrical signals to the heart 12 of the patient 14 via electrodescoupled to one or more of the leads 18, 20, 22.

The leads 18, 20, 22 extend into the heart 12 of the patient 14 to senseelectrical activity of the heart 12 and/or to deliver electricalstimulation to the heart 12. In the example shown in FIG. 1, the rightventricular (RV) lead 18 extends through one or more veins (not shown),the superior vena cava (not shown), and the right atrium 26, and intothe right ventricle 28. The left ventricular (LV) coronary sinus lead 20extends through one or more veins, the vena cava, the right atrium 26,and into the coronary sinus 30 to a region adjacent to the free wall ofthe left ventricle 32 of the heart 12. The right atrial (RA) lead 22extends through one or more veins and the vena cava, and into the rightatrium 26 of the heart 12. Lead 22 is configured to acquire signalsindicative of atrial fibrillation.

The IMD 16 may sense, among other things, electrical signals attendantto the depolarization and repolarization of the heart 12 via electrodescoupled to at least one of the leads 18, 20, 22. In some examples, theIMD 16 provides pacing therapy (e.g., pacing pulses) to the heart 12based on the electrical signals sensed within the heart 12. The IMD 16may be operable to adjust one or more parameters associated with thepacing therapy such as, e.g., pulse wide, amplitude, voltage, burstlength, etc. Further, the IMD 16 may be operable to use variouselectrode configurations to deliver pacing therapy, which may beunipolar or bipolar. The IMD 16 may also provide defibrillation therapyand/or cardioversion therapy via electrodes located on at least one ofthe leads 18, 20, 22. Further, the IMD 16 may detect arrhythmia of theheart 12, such as fibrillation of the ventricles 28, 32, and deliverdefibrillation therapy to the heart 12 in the form of electrical pulses.In some examples, IMD 16 may be programmed to deliver a progression oftherapies, e.g., pulses with increasing energy levels, until afibrillation of heart 12 is stopped.

In some examples, a programmer 24, which may be a handheld computingdevice or a computer workstation, may be used by a user, such as aphysician, technician, another clinician, and/or patient, to communicatewith the IMD 16 (e.g., to program the IMD 16). For example, the user mayinteract with the programmer 24 to retrieve information concerning oneor more detected or indicated faults associated within the IMD 16 and/orthe pacing therapy delivered therewith. The IMD 16 and the programmer 24may communicate via wireless communication using any techniques known inthe art. Examples of communication techniques may include, e.g., lowfrequency or radiofrequency (RF) telemetry, but other techniques arealso contemplated.

FIG. 2 is a conceptual diagram illustrating the IMD 16 and the leads 18,20, 22 of therapy system 10 of FIG. 1 in more detail. The leads 18, 20,22 may be electrically coupled to a therapy delivery module (e.g., fordelivery of pacing therapy), a sensing module (e.g., one or moreelectrodes to sense or monitor electrical activity of the heart 12 foruse in determining effectiveness of pacing therapy), and/or any othermodules of the IMD 16 via a connector block 34. In some examples, theproximal ends of the leads 18, 20, 22 may include electrical contactsthat electrically couple to respective electrical contacts within theconnector block 34 of the IMD 16. In addition, in some examples, theleads 18, 20, 22 may be mechanically coupled to the connector block 34with the aid of set screws, connection pins, or another suitablemechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of conductors (e.g., concentric coiledconductors, straight conductors, etc.) separated from one another byinsulation (e.g., tubular insulative sheaths). In the illustratedexample, bipolar electrodes 40, 42 are located proximate to a distal endof the lead 18. In addition, the bipolar electrodes 44, 46 are locatedproximate to a distal end of the lead 20 and the bipolar electrodes 48,50 are located proximate to a distal end of the lead 22.

The electrodes 40, 44, 48 may take the form of ring electrodes, and theelectrodes 42, 46, 50 may take the form of extendable helix tipelectrodes mounted retractably within the insulative electrode heads 52,54, 56, respectively. Each of the electrodes 40, 42, 44, 46, 48, 50 maybe electrically coupled to a respective one of the conductors (e.g.,coiled and/or straight) within the lead body of its associated lead 18,20, 22, and thereby coupled to respective ones of the electricalcontacts on the proximal end of the leads 18, 20, 22.

The electrodes 40, 42, 44, 46, 48, 50 may further be used to senseelectrical signals (e.g., morphological waveforms within electrograms(EGM)) attendant to the depolarization and repolarization of the heart12. The electrical signals are conducted to the IMD 16 via therespective leads 18, 20, 22. In some examples, the IMD 16 may alsodeliver pacing pulses via the electrodes 40, 42, 44, 46, 48, 50 to causedepolarization of cardiac tissue of the patient's heart 12. In someexamples, as illustrated in FIG. 2, the IMD 16 includes one or morehousing electrodes, such as housing electrode 58, which may be formedintegrally with an outer surface of a housing 60 (e.g.,hermetically-sealed housing) of the IMD 16 or otherwise coupled to thehousing 60. Any of the electrodes 40, 42, 44, 46, 48, 50 may be used forunipolar sensing or pacing in combination with housing electrode 58. Inother words, any of electrodes 40, 42, 44, 46, 48, 50, 58 may be used incombination to form a sensing vector, e.g., a sensing vector that may beused to evaluate and/or analysis the effectiveness of pacing therapy. Anexample of a configuration sensing and pacing may be seen with respectto U.S. patent application Ser. No. 13/717,896 filed Dec. 18, 2012, andassigned to the assignee of the present invention, the disclosure ofwhich is incorporated by reference in its entirety herein as modified bypreferably using a LVtip (i.e. electrode 46)-Rvcoil (i.e. electrode 62)for the pacing vector and the sensing vector, respectively. It isgenerally understood by those skilled in the art that other electrodescan also be selected as pacing and sensing vectors. Electrode 44 and 64refer to the third and fourth LV electrodes in the claims.

As described in further detail with reference to FIGS. 3A-3B, thehousing 60 may enclose a therapy delivery module that may include astimulation generator for generating cardiac pacing pulses anddefibrillation or cardioversion shocks, as well as a sensing module formonitoring the patient's heart rhythm. The leads 18, 20, 22 may alsoinclude elongated electrodes 62, 64, 66, respectively, which may takethe form of a coil. The IMD 16 may deliver defibrillation shocks to theheart 12 via any combination of the elongated electrodes 62, 64, 66 andthe housing electrode 58. The electrodes 58, 62, 64, 66 may also be usedto deliver cardioversion pulses to the heart 12. Further, the electrodes62, 64, 66 may be fabricated from any suitable electrically conductivematerial, such as, but not limited to, platinum, platinum alloy, and/orother materials known to be usable in implantable defibrillationelectrodes. Since electrodes 62, 64, 66 are not generally configured todeliver pacing therapy, any of electrodes 62, 64, 66 may be used tosense electrical activity during pacing therapy (e.g., for use inanalyzing pacing therapy effectiveness) and may be used in combinationwith any of electrodes 40, 42, 44, 46, 48, 50, 58. In at least oneembodiment, the RV elongated electrode 62 may be used to senseelectrical activity of a patient's heart during the delivery of pacingtherapy (e.g., in combination with the housing electrode 58 forming a RVelongated, coil, or defibrillation electrode-to-housing electrodevector).

The configuration of the exemplary therapy system 10 illustrated inFIGS. 1-2 is merely one example. In other examples, the therapy systemmay include epicardial leads and/or patch electrodes instead of or inaddition to the transvenous leads 18, 20, 22 illustrated in FIG. 1.Further, in one or more embodiments, the IMD 16 need not be implantedwithin the patient 14. For example, the IMD 16 may deliverdefibrillation shocks and other therapies to the heart 12 viapercutaneous leads that extend through the skin of the patient 14 to avariety of positions within or outside of the heart 12. In one or moreembodiments, the system 10 may utilize wireless pacing (e.g., usingenergy transmission to the intracardiac pacing component(s) viaultrasound, inductive coupling, RF, etc.) and sensing cardiac activationusing electrodes on the can/housing and/or on subcutaneous leads.

In other examples of therapy systems that provide electrical stimulationtherapy to the heart 12, such therapy systems may include any suitablenumber of leads coupled to the IMD 16, and each of the leads may extendto any location within or proximate to the heart 12. For example, otherexamples of therapy systems may include three transvenous leads locatedas illustrated in FIGS. 1-2. Still further, other therapy systems mayinclude a single lead that extends from the IMD 16 into the right atrium26 or the right ventricle 28, or two leads that extend into a respectiveone of the right atrium 26 and the right ventricle 28.

FIG. 3A is a functional block diagram of one exemplary configuration ofthe IMD 16. As shown, the IMD 16 includes a control module 81, a therapydelivery module 84 (e.g., which may include a stimulation generator), asensing module 86, and a power source 90.

The control module 81 may include a processor 80, memory 82, and atelemetry module 88. The memory 82 may include computer-readableinstructions that, when executed, e.g., by the processor 80, cause theIMD 16 and/or the control module 81 to perform various functionsattributed to the IMD 16 and/or the control module 81 described herein.Further, the memory 82 may include any volatile, non-volatile, magnetic,optical, and/or electrical media, such as a random access memory (RAM),read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasableprogrammable ROM (EEPROM), flash memory, and/or any other digital media.Memory 82 includes computer instructions related to capture management.An exemplary capture management module such as left ventricular capturemanagement (LVCM) is briefly described in U.S. Pat. No. 7,684,863, whichis incorporated by reference in its entirety. As to the delivery ofpacing stimuli, capture management algorithms typically focus onsufficient energy delivery of a pacing stimulus.

The processor 80 of the control module 81 may include any one or more ofa microprocessor, a controller, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field-programmablegate array (FPGA), and/or equivalent discrete or integrated logiccircuitry. In some examples, the processor 80 may include multiplecomponents, such as any combination of one or more microprocessors, oneor more controllers, one or more DSPs, one or more ASICs, and/or one ormore FPGAs, as well as other discrete or integrated logic circuitry. Thefunctions attributed to the processor 80 herein may be embodied assoftware, firmware, hardware, or any combination thereof.

The control module 81 may control the therapy delivery module 84 todeliver therapy (e.g., electrical stimulation therapy such as pacing) tothe heart 12 according to a selected one or more therapy programs, whichmay be stored in the memory 82. More, specifically, the control module81 (e.g., the processor 80) may control the therapy delivery module 84to deliver electrical stimulus such as, e.g., pacing pulses with theamplitudes, pulse widths, frequency, or electrode polarities specifiedby the selected one or more therapy programs (e.g., pacing therapyprograms, pacing recovery programs, capture management programs, etc.).As shown, the therapy delivery module 84 is electrically coupled toelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66, e.g., via conductorsof the respective lead 18, 20, 22, or, in the case of housing electrode58, via an electrical conductor disposed within housing 60 of IMD 16.Therapy delivery module 84 may be configured to generate and deliverelectrical stimulation therapy such as pacing therapy to the heart 12using one or more of the electrodes 40, 42, 44, 46, 48, 50, 58, 62, 64,66.

For example, therapy delivery module 84 may deliver pacing stimulus(e.g., pacing pulses) via ring electrodes 40, 44, 48 coupled to leads18, 20, and 22, respectively, and/or helical tip electrodes 42, 46, and50 of leads 18, 20, and 22, respectively. Further, for example, therapydelivery module 84 may deliver defibrillation shocks to heart 12 via atleast two of electrodes 58, 62, 64, 66. In some examples, therapydelivery module 84 may be configured to deliver pacing, cardioversion,or defibrillation stimulation in the form of electrical pulses. In otherexamples, therapy delivery module 84 may be configured deliver one ormore of these types of stimulation in the form of other signals, such assine waves, square waves, and/or other substantially continuous timesignals.

The IMD 16 may further include a switch module 85 and the control module81 (e.g., the processor 80) may use the switch module 85 to select,e.g., via a data/address bus, which of the available electrodes are usedto deliver therapy such as pacing pulses for pacing therapy, or which ofthe available electrodes are used for sensing. The switch module 85 mayinclude a switch array, switch matrix, multiplexer, or any other type ofswitching device suitable to selectively couple the sensing module 86and/or the therapy delivery module 84 to one or more selectedelectrodes. More specifically, the therapy delivery module 84 mayinclude a plurality of pacing output circuits. Each pacing outputcircuit of the plurality of pacing output circuits may be selectivelycoupled, e.g., using the switch module 85, to one or more of theelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 (e.g., a pair ofelectrodes for delivery of therapy to a pacing vector). In other words,each electrode can be selectively coupled to one of the pacing outputcircuits of the therapy delivery module using the switching module 85.

The sensing module 86 is coupled (e.g., electrically coupled) to sensingapparatus, which may include, among additional sensing apparatus, theelectrodes 40, 42, 44, 46, 48, 50, 58, 62, 64, 66 to monitor electricalactivity of the heart 12, e.g., electrocardiogram (ECG)/electrogram(EGM) signals, etc. The ECG/EGM signals may be used to analyze of aplurality of paced events. More specifically, one or more morphologicalfeatures of each paced event within the ECG/EGM signals may be used todetermine whether each paced event has a predetermined level ofeffectiveness. The ECG/EGM signals may be further used to monitor heartrate (HR), heart rate variability (HRV), heart rate turbulence (HRT),deceleration/acceleration capacity, deceleration sequence incidence,T-wave alternans (TWA), P-wave to P-wave intervals (also referred to asthe P-P intervals or A-A intervals), R-wave to R-wave intervals (alsoreferred to as the R-R intervals or V-V intervals), P-wave to QRScomplex intervals (also referred to as the P-R intervals, A-V intervals,or P-Q intervals), QRS-complex morphology, ST segment (i.e., the segmentthat connects the QRS complex and the T-wave), T-wave changes, QTintervals, electrical vectors, etc.

The switch module 85 may be also be used with the sensing module 86 toselect which of the available electrodes are used to, e.g., senseelectrical activity of the patient's heart (e.g., one or more electricalvectors of the patient's heart using any combination of the electrodes40, 42, 44, 46, 48, 50, 58, 62, 64, 66). In some examples, the controlmodule 81 may select the electrodes that function as sensing electrodesvia the switch module within the sensing module 86, e.g., by providingsignals via a data/address bus. In some examples, the sensing module 86may include one or more sensing channels, each of which may include anamplifier.

In some examples, sensing module 86 includes a channel that includes anamplifier with a relatively wider pass band than the R-wave or P-waveamplifiers. Signals from the selected sensing electrodes that areselected for coupling to this wide-band amplifier may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 82 as an EGM. In someexamples, the storage of such EGMs in memory 82 may be under the controlof a direct memory access circuit. The control module 81 (e.g., usingthe processor 80) may employ digital signal analysis techniques tocharacterize the digitized signals stored in memory 82 to analyze and/orclassify one or more morphological waveforms of the EGM signals todetermine pacing therapy effectiveness. For example, the processor 80may be configured to determine, or obtain, one more feature of one ormore sensed morphological waveforms within one of more electricalvectors of the patient's heart and store the one or more features withinthe memory 82 for use in determining effectiveness of pacing therapy ata later time. When a patient is experiencing AF, processor 80 is furtherconfigured to determine pacing effectiveness using a normal process modeor a duty cycle mode.

If IMD 16 is configured to generate and deliver pacing pulses to theheart 12, the control module 81 may include a pacer timing and controlmodule, which may be embodied as hardware, firmware, software, or anycombination thereof. The pacer timing and control module may include oneor more dedicated hardware circuits, such as an ASIC, separate from theprocessor 80, such as a microprocessor, and/or a software moduleexecuted by a component of processor 80, which may be a microprocessoror ASIC. The pacer timing and control module may include programmablecounters which control the basic time intervals associated with DDD,VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and othermodes of single and dual chamber pacing. In the aforementioned pacingmodes, “D” may indicate dual chamber, “V” may indicate a ventricle, “I”may indicate inhibited pacing (e.g., no pacing), and “A” may indicate anatrium. The first letter in the pacing mode may indicate the chamberthat is paced, the second letter may indicate the chamber in which anelectrical signal is sensed, and the third letter may indicate thechamber in which the response to sensing is provided.

Intervals defined by the pacer timing and control module within controlmodule 81 may include atrial and ventricular pacing escape intervals,refractory periods during which sensed P-waves and R-waves areineffective to restart timing of the escape intervals, and/or the pulsewidths of the pacing pulses. As another example, the pacer timing andcontrol module may define a blanking period, and provide signals fromsensing module 86 to blank one or more channels, e.g., amplifiers, for aperiod during and after delivery of electrical stimulation to the heart12. The durations of these intervals may be determined in response tostored data in memory 82. The pacer timing and control module of thecontrol module 81 may also determine the amplitude of the cardiac pacingpulses.

During pacing, escape interval counters within the pacer timing/controlmodule may be reset upon sensing of R-waves and P-waves. Therapydelivery module 84 (e.g., including a stimulation generator) may includeone or more pacing output circuits that are coupled, e.g., selectivelyby the switch module 85, to any combination of electrodes 40, 42, 44,46, 48, 50, 58, 62, or 66 appropriate for delivery of a bipolar orunipolar pacing pulse to one of the chambers of heart 12. The controlmodule 81 may reset the escape interval counters upon the generation ofpacing pulses by therapy delivery module 84, and thereby control thebasic timing of cardiac pacing functions, including anti-tachyarrhythmiapacing.

In some examples, the control module 81 may operate as an interruptdriven device, and may be responsive to interrupts from pacer timing andcontrol module, where the interrupts may correspond to the occurrencesof sensed P-waves and R-waves and the generation of cardiac pacingpulses. Any necessary mathematical calculations may be performed by theprocessor 80 and any updating of the values or intervals controlled bythe pacer timing and control module may take place following suchinterrupts. A portion of memory 82 may be configured as a plurality ofrecirculating buffers, capable of holding series of measured intervals,which may be analyzed by, e.g., the processor 80 in response to theoccurrence of a pace or sense interrupt to determine whether thepatient's heart 12 is presently exhibiting atrial or ventriculartachyarrhythmia.

The telemetry module 88 of the control module 81 may include anysuitable hardware, firmware, software, or any combination thereof forcommunicating with another device, such as the programmer 24 asdescribed herein with respect to FIG. 1. For example, under the controlof the processor 80, the telemetry module 88 may receive downlinktelemetry from and send uplink telemetry to the programmer 24 with theaid of an antenna, which may be internal and/or external. The processor80 may provide the data to be uplinked to the programmer 24 and thecontrol signals for the telemetry circuit within the telemetry module88, e.g., via an address/data bus. In some examples, the telemetrymodule 88 may provide received data to the processor 80 via amultiplexer. In at least one embodiment, the telemetry module 88 may beconfigured to transmit an alarm, or alert, if the pacing therapy becomesineffective or less effective (e.g., does not have a predetermined levelof effectiveness).

The various components of the IMD 16 are further coupled to a powersource 90, which may include a rechargeable or non-rechargeable battery.A non-rechargeable battery may be selected to last for several years,while a rechargeable battery may be inductively charged from an externaldevice, e.g., on a daily or weekly basis.

FIG. 3B is yet another embodiment of a functional block diagram for IMD16. FIG. 3B depicts bipolar RA lead 22, bipolar RV lead 18, and bipolarLV CS lead 20 without the LA CS pace/sense electrodes 28 and 30 coupledwith an IPG circuit 31 having programmable modes and parameters of abi-ventricular DDD/R type known in the pacing art. In turn, the sensorsignal processing circuit 49 indirectly couples to the timing circuit 83and via data and control bus to microcomputer circuitry 33. Optionally,sensor signal process 49 is coupled to another sensor such as aoxygenation sensors, pressure sensors, pH sensors and respirationsensors etc. The IPG circuit 31 is illustrated in a functional blockdiagram divided generally into a microcomputer circuit 33 and a pacingcircuit 83. The pacing circuit includes the digital controller/timercircuit 83, the output amplifiers circuit 51, the sense amplifierscircuit 55, the RF telemetry transceiver 41, the activity sensor circuit35 as well as a number of other circuits and components described below.

Crystal oscillator circuit 47 provides the basic timing clock for thepacing circuit 320, while battery 29 provides power. Power-on-resetcircuit 45 responds to initial connection of the circuit to the batteryfor defining an initial operating condition and similarly, resets theoperative state of the device in response to detection of a low batterycondition. Reference mode circuit 37 generates stable voltage referenceand currents for the analog circuits within the pacing circuit 320,while analog to digital converter ADC and multiplexer circuit 39digitizes analog signals and voltage to provide real time telemetry if acardiac signals from sense amplifiers 55, for uplink transmission via RFtransmitter and receiver circuit 41. Voltage reference and bias circuit37, ADC and multiplexer 39, power-on-reset circuit 45 and crystaloscillator circuit 47 may correspond to any of those presently used incurrent marketed implantable cardiac pacemakers.

If the IPG is programmed to a rate responsive mode, the signals outputby one or more physiologic sensor are employed as a rate controlparameter (RCP) to derive a physiologic escape interval. For example,the escape interval is adjusted proportionally the patient's activitylevel developed in the patient activity sensor (PAS) circuit 35 in thedepicted, exemplary IPG circuit 31. The patient activity sensor 27 iscoupled to the IPG housing and may take the form of a piezoelectriccrystal transducer as is well known in the art and its output signal isprocessed and used as the RCP. Sensor 27 generates electrical signals inresponse to sensed physical activity that are processed by activitycircuit 35 and provided to digital controller/timer circuit 83. Activitycircuit 35 and associated sensor 27 may correspond to the circuitrydisclosed in U.S. Pat. Nos. 5,052,388 and 4,428,378. Similarly, thepresent invention may be practiced in conjunction with alternate typesof sensors such as oxygenation sensors, pressure sensors, pH sensors andrespiration sensors, all well known for use in providing rate responsivepacing capabilities. Alternately, QT time may be used as the rateindicating parameter, in which case no extra sensor is required.Similarly, the present invention may also be practiced in non-rateresponsive pacemakers.

Data transmission to and from the external programmer is accomplished bymeans of the telemetry antenna 57 and an associated RF transceiver 41,which serves both to demodulate received downlink telemetry and totransmit uplink telemetry. Uplink telemetry capabilities will typicallyinclude the ability to transmit stored digital information, e.g.operating modes and parameters, EGM histograms, and other events, aswell as real time EGMs of atrial and/or ventricular electrical activityand Marker Channel pulses indicating the occurrence of sensed and paceddepolarizations in the atrium and ventricle, as are well known in thepacing art.

Microcomputer 33 contains a microprocessor 80 and associated systemclock and on-processor RAM and ROM chips 82A and 82B, respectively. Inaddition, microcomputer circuit 33 includes a separate RAM/ROM chip 82Cto provide additional memory capacity. Microprocessor 80 normallyoperates in a reduced power consumption mode and is interrupt driven.Microprocessor 80 is awakened in response to defined interrupt events,which may include A-TRIG, RV-TRIG, LV-TRIG signals generated by timersin digital timer/controller circuit 83 and A-EVENT, RV-EVENT, andLV-EVENT signals generated by sense amplifiers circuit 55, among others.The specific values of the intervals and delays timed out by digitalcontroller/timer circuit 83 are controlled by the microcomputer circuit33 by means of data and control bus 306 from programmed-in parametervalues and operating modes. In addition, if programmed to operate as arate responsive pacemaker, a timed interrupt, e.g., every cycle or everytwo seconds, may be provided in order to allow the microprocessor toanalyze the activity sensor data and update the basic A-A, V-A, or V-Vescape interval, as applicable. In addition, the microprocessor 80 mayalso serve to define variable, operative AV delay intervals and theenergy delivered to each ventricle.

In one embodiment, microprocessor 80 is a custom microprocessor adaptedto fetch and execute instructions stored in RAM/ROM unit 82C in aconventional manner. It is contemplated, however, that otherimplementations may be suitable to practice the present invention. Forexample, an off-the-shelf, commercially available microprocessor ormicrocontroller, or custom application-specific, hardwired logic, orstate-machine type circuit may perform the functions of microprocessor80.

Digital controller/timer circuit 83 operates under the general controlof the microcomputer 33 to control timing and other functions within thepacing circuit 320 and includes a set of timing and associated logiccircuits of which certain ones pertinent to the present invention aredepicted. The depicted timing circuits include URI/LRI timers 83A, V-Vdelay timer 83B, intrinsic interval timers 83C for timing elapsedV-EVENT to V-EVENT intervals or V-EVENT to A-EVENT intervals or the V-Vconduction interval, escape interval timers 83D for timing A-A, V-A,and/or V-V pacing escape intervals, an AV delay interval timer 83E fortiming the atrial-left ventricular pace (A-LVp) delay (or atrial rightventricular pace (A-RVp delay) from a preceding A-EVENT or A-TRIG, apost-ventricular timer for timing post-ventricular time periods, and adate/time clock 83G.

The AV delay interval timer 83E is loaded with an appropriate delayinterval for one ventricular chamber (i.e., either an A-RVp delay or anA-LVp delay as determined using known methods) to time-out starting froma preceding A-PACE or A-EVENT. The interval timer 83E triggers pacingstimulus delivery, and can based on one or more prior cardiac cycles (orfrom a data set empirically derived for a given patient).

The post-event timers 83F time out the post-ventricular time periodsfollowing an RV-EVENT or LV-EVENT or a RV-TRIG or LV-TRIG andpost-atrial time periods following an A-EVENT or A-TRIG. The durationsof the post-event time periods may also be selected as programmableparameters stored in the microcomputer 33. The post-ventricular timeperiods include the PVARP, a post-atrial ventricular blanking period(PAVBP), a ventricular blanking period (VBP), a post-ventricular atrialblanking period (PVARP) and a ventricular refractory period (VRP)although other periods can be suitably defined depending, at least inpart, on the operative circuitry employed in the pacing engine. Thepost-atrial time periods include an atrial refractory period (ARP)during which an A-EVENT is ignored for the purpose of resetting any AVdelay, and an atrial blanking period (ABP) during which atrial sensingis disabled. It should be noted that the starting of the post-atrialtime periods and the AV delays can be commenced substantiallysimultaneously with the start or end of each A-EVENT or A-TRIG or, inthe latter case, upon the end of the A-PACE which may follow the A-TRIG.Similarly, the starting of the post-ventricular time periods and the V-Aescape interval can be commenced substantially simultaneously with thestart or end of the V-EVENT or V-TRIG or, in the latter case, upon theend of the V-PACE which may follow the V-TRIG. The microprocessor 80also optionally calculates AV delays, post-ventricular time periods, andpost-atrial time periods that vary with the sensor based escape intervalestablished in response to the RCP(s) and/or with the intrinsic atrialrate.

The output amplifiers circuit 51 contains a RA pace pulse generator (anda LA pace pulse generator if LA pacing is provided), a RV pace pulsegenerator, and a LV pace pulse generator or corresponding to any ofthose presently employed in commercially marketed cardiac pacemakersproviding atrial and ventricular pacing. In order to trigger generationof an RV-PACE or LV-PACE pulse, digital controller/timer circuit 83generates the RV-TRIG signal at the time-out of the A-RVp delay (in thecase of RV pre-excitation) or the LV-TRIG at the time-out of the A-LVpdelay (in the case of LV pre-excitation) provided by AV delay intervaltimer 83E (or the V-V delay timer 83B). Similarly, digitalcontroller/timer circuit 83 generates an RA-TRIG signal that triggersoutput of an RA-PACE pulse (or an LA-TRIG signal that triggers output ofan LA-PACE pulse, if provided) at the end of the V-A escape intervaltimed by escape interval timers 83D.

The output amplifiers circuit 51 includes switching circuits forcoupling selected pace electrode pairs from among the lead conductorsand indifferent electrodes (IND) to the RA pace pulse generator (and LApace pulse generator if provided), RV pace pulse generator and LV pacepulse generator. Indifferent electrode means any electrode that has nointeraction with a designated element. For example there is nointeraction between the atrial electrodes and the LV electrode (i.e. nopacing, sensing, or even sub-threshold measurements) since that pathwayhas no value. If a RV electrode can interact with the LV electrode, thenthe RV electrode cannot be defined as being indifferent unlessspecifically defined as isolated from the LV electrode.

Pace/sense electrode pair selection and control circuit 53 selects leadconductors and associated pace electrode pairs to be coupled with theatrial and ventricular output amplifiers within output amplifierscircuit 51 for accomplishing RA, LA, RV and LV pacing.

The sense amplifiers circuit 55 contains sense amplifiers correspondingto any of those presently employed in contemporary cardiac pacemakersfor atrial and ventricular pacing and sensing. As noted in theabove-referenced, commonly assigned, '324 patent, it has been common inthe prior art to use very high impedance P-wave and R-wave senseamplifiers to amplify the voltage difference signal which is generatedacross the sense electrode pairs by the passage of cardiacdepolarization wavefronts. The high impedance sense amplifiers use highgain to amplify the low amplitude signals and rely on pass band filters,time domain filtering and amplitude threshold comparison to discriminatea P-wave or R-wave from background electrical noise. Digitalcontroller/timer circuit 83 controls sensitivity settings of the atrialand ventricular sense amplifiers 55.

The sense amplifiers are typically uncoupled from the sense electrodesduring the blanking periods before, during, and after delivery of a pacepulse to any of the pace electrodes of the pacing system to avoidsaturation of the sense amplifiers. The sense amplifiers circuit 55includes blanking circuits for uncoupling the selected pairs of the leadconductors and the IND_CAN electrode on lead 20 from the inputs of theRA sense amplifier (and LA sense amplifier if provided), RV senseamplifier and LV sense amplifier during the ABP, PVABP and VBP. Thesense amplifiers circuit 55 also includes switching circuits forcoupling selected sense electrode lead conductors and the IND_CANelectrode on lead 20 to the RA sense amplifier (and LA sense amplifierif provided), RV sense amplifier and LV sense amplifier. Again, senseelectrode selection and control circuit 53 selects conductors andassociated sense electrode pairs to be coupled with the atrial andventricular sense amplifiers within the output amplifiers circuit 51 andsense amplifiers circuit 55 for accomplishing RA, LA, RV and LV sensingalong desired unipolar and bipolar sensing vectors.

Right atrial depolarizations or P-waves in the RA-SENSE signal that aresensed by the RA sense amplifier result in a RA-EVENT signal that iscommunicated to the digital controller/timer circuit 83. Similarly, leftatrial depolarizations or P-waves in the LA-SENSE signal that are sensedby the LA sense amplifier, if provided, result in a LA-EVENT signal thatis communicated to the digital controller/timer circuit 83. Ventriculardepolarizations or R-waves in the RV-SENSE signal are sensed by aventricular sense amplifier result in an RV-EVENT signal that iscommunicated to the digital controller/timer circuit 83. Similarly,ventricular depolarizations or R-waves in the LV-SENSE signal are sensedby a ventricular sense amplifier result in an LV-EVENT signal that iscommunicated to the digital controller/timer circuit 83. The RV-EVENT,LV-EVENT, and RA-EVENT, LA-SENSE signals may be refractory ornon-refractory, and can inadvertently be triggered by electrical noisesignals or aberrantly conducted depolarization waves rather than trueR-waves or P-waves.

A pacing (e.g., for LV and/or BV pacing) ratio or percentage, which isthe number of paced heart beats divided by the total number of heartbeats, often expressed as a percentage of the total number of heartbeats, may be a useful metric for evaluating the effectiveness of CRTbut, there are cases in which the pacing ratio or percentage ismisleading. For example, in some scenarios such as when pseudo-fusion(e.g., ventricular pacing fails to properly alter electrical activationpatterns) is occurring, a high pacing ratio or percentage may notnecessarily mean that CRT is effective. Automatic beat-to-beat analysisof the evoked response (e.g., paced QRS complexes) in monitored EGMsignals may be used to determine whether the paced heartbeat waseffectively paced, and hence, provides more resolution to a pacingratio. For example, the heartbeats that were paced but determined to notbe effectively paced (e.g., depending on the degree of fusion betweenintrinsic and paced activation, etc.) may be excluded from the pacingratio thereby providing a more accurate metric of pacing efficacy and/orefficiency, which may referred to as a pacing effectiveness ratio.

A feature-based classification may enable beat-to-beat rhythmclassification in a device (e.g., IMD 16) employing cardiac pacing(e.g., CRT pacing such as left ventricular fusion pacing, biventricularpacing (BiV), multisite LV pacing etc.) and may add value to the deviceby providing useful diagnostic indices to a physician. The computationalprice involved in such feature-based beat-to-beat classifications may beminimal and may be implemented within the architecture of devices suchas the IMD 16 described herein with reference to FIGS. 1-3B. Forexample, the exemplary methods described herein may combine algebraicoperations and comparisons and/or may require a single normalization perbeat compared to multiple intensive mathematical operations andnormalizations that are often required for detailed template matchingalgorithms.

The present disclosure is directed to increasing a patient's percent ofeffective CRT pacing during AF without significantly increasingventricular rate. FIG. 4 depicts a normal process mode 900 which iscontinuously implemented to modify IMD 16 pacing rate during AF. Normalprocess mode 900 begins at block 902 in which the process 900 waits forthe detection of a next ventricular event. At block 904, after the nextventricular event has been detected, a determination is made as towhether the ventricular event was a sensed ventricular event or a pacedventricular event. IMD 16 can automatically determine sensed from pacedventricular events by tracking whether the signal is acquired from anelectrode used to sense or pace at a particular time. Paced ventricularevents follow the paced path to block 906 in which an EGM is acquired.It can be advantageous for the EGM to be acquired between the LV pacingcathode and a distant (i.e. indifferent) electrode like the RV coil orthe device case (i.e. housing electrode 58). Generation of EGM data isdescribed in U.S. Pat. No. 5,776,168 which is incorporated by referencein its entirety. Other methods of obtaining EGM data can also be used.

At decision block 908, the processor 80 uses EGM data to determinewhether effective LV capture has occurred in response to the sensedventricular event. U.S. patent application Ser. No. 13/707,366, filedDec. 6, 2012 and entitled Effective Capture Test, incorporated byreference in its entirety herein, discloses exemplary processes thatrely on the EGM data to determine whether pacing stimulus is effectivelycapturing a ventricle. Other exemplary processes that can be used todetermine effective capture include U.S. Pat. No. 6,044,296. If there isnot effective LV capture, the NO path from block 908 continues to block920 which causes a HR target level to be increased by Delta3. Exemplaryvalues for Delta3 are presented below in Table 1. The HR target level isthe desired or ideal HR goal for a patient. If the patient has decreasedcardiac capacity, the target HR may not be within a normal HR range.

Block 928 ensures that the HR target level stays within the upper limitprogrammed into IMD 16. At decision block 928, a determination is thenmade as to whether the HR is greater than an upper limit. HR is theactual HR sensed by IMD 16 from a patient at a particular time. Theupper limit is related to the upper limit of the programmable pacingrate. For a patient, the upper limit can be predetermined or determinedby using data dynamically updated from physiological data from thepatient. The upper limit can range from about 100 BPM to about 150 BPM.If the HR is greater than an upper limit, the YES path continues toblock 930 and the HR target level is then set at the upper limit level.Thereafter, the RETURN path continues to block 902. If the HR is lessthan or equal to the upper limit, the NO path from block 928 returns toblock 902.

Returning to block 908, if the paced ventricular event provideseffective LV capture, then the YES path from block 908 continues todecision block 910. At block 910, a determination is made as to whetherthe prior ventricular event had effective LV capture. The priorventricular event is defined as the ventricular event that wasimmediately before the “next ventricular event” from block 902. If theprior ventricular event had also effectively captured the LV, the YESpath continues to block 912 in which the HR target level is decreased byDelta1. Exemplary values for Delta1 are listed in Table 1.

If the prior ventricular event did not provide effective LV capture,then the NO path from block 910 decreases the HR target level by Delta2at block 914. Exemplary values for Delta2 is presented below in Table 1.

Block 916, like block 928, ensures that the HR stays within theprogrammed limits into IMD 16. In particular, a determination is made asto whether the HR is lower than the lower limit at decision block 916.Generally, the lower limit is set to the sensor rate (i.e., the HRdetermined by the activity sensor in rate-responsive pacing modes).Alternatively, the lower limit could be set to the programmed lower ratelimit of the IMD 16. The programmed lower rate limit is typically set to40-70 BPM. The lower rate can also be impacted by “sleep mode” asdescribed in the art. Sleep mode for IMD 16 can be configured to dropthe lower rate at night.

If the HR is below the lower limit, then the HR target level is set tothe lower limit at block 918. If the HR is equal to or above the lowerlimit, then the HR target level remains unchanged and the NO path fromblock 916 returns to block 902 to wait for the next ventricular event.

Returning to block 904, if a sensed ventricular event has occurredrather than a paced ventricular event, then a determination is made atdecision block 922 as to whether the preceding beat was sensed. If thepreceding beat was not sensed, the NO path from decision block 922requires that the HR target level be increased by Delta4 at block 924.If the preceding beat was sensed, the YES path from decision block 922requires that the HR target level be increased by Delta5 at block 926.The logic of block 928, previously explained, determines whether thesensed ventricular event has the HR target level set to the upper limitor the RETURN path to block 902 is followed.

In the exemplary table presented below, the “Deltas” refer to the aboveflow diagram for the normal operation mode 900. The Deltas are functionsof heart rate thereby allowing the algorithm to become less “aggressive”at higher heart rates. As previously explained, the heart rate, at theend of the initialization period, is augmented by a Delta value. TheDelta value selected from Table 1 depends upon the patient's heart ratedetermined by IMD 16. IMD 16 ensures that the HR is analyzed for asufficient period of time to achieve a relatively consistent reading ofthe HR during real-time. The HR is then used to determine theappropriate Delta to adjust the HR target level.

Deltas1 and 2 involve decreases to the HR target level while Deltas 3,4, and 5 involve increases to the HR target level.

TABLE 1 exemplary Delta values HR change as a funcTion of HR HR in arange between 70 BPM to Delta values HR > 90 BPM 90 BPM HR < 70 BPMDelta1 (BPM) 1 2 2 Delta2 (BPM) 0.5 0.5 1 Delta3 (BPM) 1 2 3 Delta4(BPM) 1 2 3 Delta5 (BPM) 1 3 5

While values for each Delta is found in the table, it is generallyunderstood that other values may also be used. In one or moreembodiments, the ratios between each Delta can be useful in determiningthe HR target level.

FIG. 6 depicts an exemplary set of sequential beat signals that mayoccur during a normal mode of operation described in the flow diagram ofFIG. 4. The sequential beats show differences in the evoked responsesfollowing delivery of a pace or the intrinsic activation following asensing of a ventricular event, as observed in the electrogram sensedbetween the LV pacing cathode and an indifferent electrode like the RVcoil or device case. The Y-axis depicts the amplitude while the X-axisdepicts the time, measured in seconds, after delivering a pacingstimulus or sensing a ventricular event. Signals 1100 and 1102 areindicative of pacing stimulus that effectively captures a ventricle. Theobservation of two consecutively effective capture beats results in adecrease in the HR target level by Delta1. Signal 1104 is indicative ofa sensed ventricular event. Observation of a sensed ventricular eventthat is preceded by an effective capture event results in an increase inthe HR target level by Delta4. Signal 1106 is indicative of a sensedventricular event. Observation of two consecutively sensed eventsresults in an increase in the HR target level by Delta5. Signal 1108 isindicative of a paced ventricular event with ineffective capture, whichresults in an increase in the HR target level by Delta3. Signal 1110 isindicative of a paced ventricular event with effective capture. Signal1110 resulted in a decrease in the HR target level by Delta2. Signal1112 is indicative of a paced ventricular event with effective capture.This resulted in a decrease in the HR target level by Delta1.

While FIGS. 4 and 6 relate to normal process mode, a duty cycle modethat conserves battery life may also be used to modulate the pacing ratein response to observation of effective capture during AF. Duty cyclemode means that the IMD 16 is not continuously verifying whethereffective capture of a ventricle is occurring during AF. Specifically,duty cycle mode means that verification of effective capture during AFonly occurs a certain percentage of the time. For example, the dutycycle mode may operate 10% percent of the time (e.g. minutes, hours,daily, weekly, etc.). Alternatively, the duty-cycle mode of operationcan be configured as a function of AF burden. For example, the dutycycle mode can operate a certain number of hours while AF is detected.In one or more embodiments, duty cycle mode can be configured as afunction of beat numbers (i.e., 10 beats out of 40) or seconds (i.e., 10seconds out of 40 seconds).

In yet another embodiment, a combination of the normal mode of operationand the duty cycle mode of operation can be employed. A normal mode ofoperation can be used for the first N hours of AF per day where N is anynumber between 1 and 12 followed by a duty-cycle mode. A fixed oradjustable duty-cycle mode of X % can be then employed by the IMD. Theduty-cycle mode of X % means that the feature, related to determiningeffective capture during AF, is activated in the duty cycle mode suchthat some beats are actively monitored every minute or every 30 seconds(e.g. 10 beats of every 40 beats, 10 beats of every 30 beats etc.). Inone or more embodiments, the feature, related to modulating the pacingrate in response to observation of effective capture during AF, is notturned off several minutes at a time.

In yet another embodiment, the first day IMD 16 operates in the normalmode (i.e. continuously “on” or activated) for a day or more and thenswitches to fixed or adjustable dual cycle mode. Fixed dual cycle timemeans that the operation that determines effective capture during AFoccurs over a consistent amount of time.

In still yet another embodiment, the duty cycle mode can operate apredetermined percentage of time over each hour. For example, the dutycycle mode can operate 50% of an hour; therefore, the duty cycle modewould run, for example, a certain number of beats and then off for acertain number of beats. More specifically, a 50% duty cycle mode isexecuted as 10 beats on, 10 beats off and not 30 minutes on and 30minutes off.

Operations of duty cycle mode are similar to the normal cycle modeexcept as modified therein. Process 1000 of FIG. 5 begins at block 1002in which a predetermined amount of consecutive beats (e.g. 30 beatsetc.) are analyzed. Based upon the analysis, the HR target level can beadjusted according to the flow diagram of FIG. 4 which outlines theoperation for the normal operation mode. For example, the HR targetlevel is increased by 1 BPM if the patient's detected HR is greater than100 BPM. In one or more other embodiments, the HR target level isincreased by +2 BPM if the patient's HR is determined to be between 80BPM and 100 BPM. At block 1004, the final HR target level can beincreased by determining application of the conditions below. Forexample, the HR target level can be increased by a predetermined amountsuch as“1” if the beats per minute (BPM) is greater than 100 BPM.Alternatively, the HR target level can be increased by anotherpredetermined amount such as “3” if the BPM is greater than 80 BPM butless than 100 BPM. In yet another embodiment, the HR target level can beincreased by yet another predetermined amount such as “5” if the BPM) isgreater than 100 BPM. Table 2 summarizes the BPM target level based uponthe presently or latest sensed BPM from the patient. Table 2 applies toadjustments made to the HR target level at block 1004.

TABLE 2 Increase BPM target level based upon the presently or latestsensed BPM Adjustment to Exemplary HR detected BPM target adjustment toin patient level BPM target level HR > 100 BPM X X = 1 80 < HR < 100 X +2 3 HR < 80 BPM X + 4 5

Block 1006 represents a number of operations. For example, the currentHR target level is set to the current or latest HR target level valuestored in memory. Additionally, the analysis of effective capture isdisabled to allow the HR target level to be increased until theventricular event counter equals a predetermined integer J. By checkinga predetermined number of ventricular events, the data obtained as theresult of the ventricular events ensures greater consistency in thedata. In one or more embodiments, integer J can be any integer such as30 or less. In one or more other embodiments, J can be set to 20 orless. In yet another embodiment, J=10 or less. In addition to settingthe value of integer J, the sensed ventricular event counter, M, canalso be set to a predetermined value (e.g. 0, 1 etc.).

At block 1008, process 1000 waits for the next ventricular event to besensed. After the next ventricular event has been sensed, the totalventricular events (i.e. sensed and paced), represented by N, isincremented at block 1010. M, the counter representing sensedventricular events (i.e. not paced ventricular events), is onlyincremented when a sensed event is detected at block 1010.

At block 1012, a determination is made as to whether a ventricular eventcounter is equal to a predetermined integer J. J is the total number ofventricular events to be counted. If a ventricular event counter is notequal to a predetermined integer J, the NO path from block 1012continues to decision block 1014. Block 1014 determines whether thepacing pulse is effectively capturing the ventricle(s) by comparing thesensed ventricular beats to a predetermined integer referred to as C1(e.g. 2, 4, or 6, etc.). If M is greater than or equal to C1, then theHR target level the YES path from block 1014 continues to block 1016 inwhich the HR target level is increased by 2 BPM. The NO path from block1014 returns to block 1008 to wait for the next ventricular event.

The YES path from block 1012 continues to block 1018 in which effectivecapture analysis is enabled. Effective capture analysis can be performedon any or all EGM data stored into memory. At decision block 1020, adetermination is made as to whether effective capture is occurring withthe next predetermined number C3 (e.g. 10 etc.) ventricular events. Atblock 1022, the HR target level is modified according to the number ofeffectively captured consecutive beats (e.g. 5-10). For example, if thenumber of effectively captured beats is less than 3 beats out of 10then, the HR target level is increased by +3 BPM. In another example, ifthe number of effectively captured beats is between 3 and 6 beats (i.e.3<beats<6) out of 10 beats, then the HR target level is increased by +2BPM. In yet another example, if the beats equal 6 or 7 out of 10 beats,then the HR target level is increased by +1 BPM. In yet another example,if the effectively captured beats equal 8 out of 10 beat, the HR targetlevel remains unchanged. In still yet another example, if theeffectively captured beats equal 9 out of 10 beats, then the HR targetlevel is decreased by −2 BPM. In yet another example, if the numbereffectively captured beats is 10 out of 10 beats, then the HR targetlevel is decreased by −3 BPM.

In one or more preferred embodiments, a non-duty cycle mode is used forthe first 30 seconds (i.e., an “initialization period”) to adjust theheart rate during AF. The heart rate at the end of the initializationperiod is then augmented by 1 BPM if the heart rate is greater than 100BPM, 3 BPM if the heart rate is greater than 80 BPM but less than 100BPM, and 5 BPM if the heart rate is less than 80 BPM. This finaldesignated pacing rate is then applied during a hold period of 30seconds. During the hold period, the pacing rate can only be adjustedupward if 6 of 10 ventricular events in any period of 10 ventricularevents are sensed, not paced. If this occurs, the heart rate isincreased by 2 BPM and the buffer of 10 beats is cleared. Battery poweris conserved because determination of sensed versus paced is very simplecompared to the determination of effective capture versus ineffectivecapture. At the end of the hold period, 10 beats are analyzed foreffective capture. Depending on how many of the 10 beats have effectivecapture, the pacing rate for the next 30 second hold period is adjustedup or down.

The techniques described in this disclosure, including those attributedto the IMD 16, the programmer 24, or various constituent components, maybe implemented, at least in part, in hardware, software, firmware, orany combination thereof. For example, various aspects of the techniquesmay be implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices, or other devices.The term “module,” “processor,” or “processing circuitry” may generallyrefer to any of the foregoing logic circuitry, alone or in combinationwith other logic circuitry, or any other equivalent circuitry.

Such hardware, software, and/or firmware may be implemented within thesame device or within separate devices to support the various operationsand functions described in this disclosure. In addition, any of thedescribed units, modules, or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed by one or moreprocessors to support one or more aspects of the functionality describedin this disclosure.

Listed below are co-pending U.S. patent applications that describevarious aspects of the apparatus and methods described herein. Theco-pending applications are incorporated by reference in theirentireties.

Co-pending U.S. patent application Ser. No. 13/707,366 entitled“EFFECTIVE CAPTURE” filed by Subham Ghosh et al. and assigned to thesame assignee of the present disclosure.

Co-pending U.S. patent application Ser. No. 13/707,391 entitled“EFFECTIVE CAPTURE” filed by Subham Ghosh et al. and assigned to thesame assignee of the present disclosure.

Co-pending U.S. patent application Ser. No. 13/707,440 entitled“EFFECTIVE CAPTURE” filed by Subham Ghosh et al. and assigned to thesame assignee of the present disclosure.

Co-pending U.S. patent application Ser. No. 13/707,458 entitled“EFFECTIVE CAPTURE” filed by Subham Ghosh et al. and assigned to thesame assignee of the present disclosure.

Co-pending U.S. patent application Ser. No. 13/772,840 entitled“CRITERIA FOR OPTIMAL ELECTRICAL RESYCHRONIZATION DURING FUSION PACING”filed by Subham Ghosh et al. and assigned to the same assignee of thepresent disclosure.

This disclosure has been provided with reference to illustrativeembodiments and is not meant to be construed in a limiting sense. Asdescribed previously, one skilled in the art will recognize that othervarious illustrative applications may use the techniques as describedherein to take advantage of the beneficial characteristics of theapparatus and methods described herein. Various modifications of theillustrative embodiments, as well as additional embodiments of thedisclosure, will be apparent upon reference to this description.

What is claimed:
 1. An apparatus for determining whether ventricularpacing stimuli are capturing a paced ventricle during atrialfibrillation, comprising: processing means for determining whether apatient is experiencing atrial fibrillation; delivering means fordelivering the ventricular pacing stimuli in response to determining thepatient is in atrial fibrillation; sensing means for sensing signals inresponse to the ventricular pacing stimuli; processing means fordetermining whether the ventricular pacing stimuli are effectivelycapturing the paced ventricle over a series of the delivered ventricularpacing stimuli, the determination of whether the ventricular pacing areeffectively capturing the paced ventricle comprising determining anumber of consecutive ones of the series that effectively capture thepaced ventricle; and modifying means for modifying a pacing rate duringatrial fibrillation in response to the determination of effectivecapture by the ventricular pacing stimuli delivered during the series;wherein the modification to the pacing rate is based upon the determinednumber of consecutive ones of the series that effectively captured thepaced ventricle.
 2. The apparatus of claim 1 wherein the processingmeans for determining whether the ventricular pacing stimuli arecapturing the paced ventricle is periodically performed to maximizebattery life.
 3. The apparatus of claim 2 wherein periodicallydetermining whether the ventricular pacing stimuli are capturing thepaced ventricle occurs solely during atrial fibrillation.
 4. Theapparatus of claim 1 wherein the ventricular pacing stimuli are leftventricular (LV) occurring at a LV tip electrode, and a monitoringvector is selected from one of a Lvtip-Rvcoil or a Lvtip-device case. 5.The apparatus of claim 1 wherein the ventricular pacing stimuli are LVpacing occurring at an LV ring, and a monitoring vector is selected fromone of a LVring-Rvcoil or a LVring-device case.
 6. The apparatus ofclaim 1 wherein the ventricular pacing stimuli are LV pacing occurringat a third LV electrode, and a monitoring vector is selected from one ofthe third LVelectrode-Rvcoil or third LVelectrode-device case.
 7. Theapparatus of claim 1 wherein the ventricular pacing stimuli are LVpacing occurring at a fourth LV electrode, and a monitoring vector isselected from one of the fourth LVelectrode-Rvcoil or fourthLVelectrode-device case.
 8. The apparatus of claim 1 wherein determiningwhether a ventricular pacing stimuli are effectively capturing a pacedventricle occurs during normal device pace timing operation.
 9. Theapparatus of claim 1 wherein a duty cycle mode is employed whendetermining whether effective ventricular capture is occurring duringAF.
 10. The apparatus of claim 1 further comprising means fordetermining whether a ventricular event is a paced ventricular event.11. The apparatus of claim 10 further comprising means for acquiring anEGM in response to determining a ventricular event is a pacedventricular event.
 12. The apparatus of claim 1 wherein the ventricularpacing stimuli are delivered during one of biventricular pacing,multisite left ventricular pacing and fusion pacing.
 13. A method fordetermining whether ventricular pacing stimuli are capturing a pacedventricle during atrial fibrillation, comprising: determining whether apatient is experiencing atrial fibrillation; delivering the ventricularpacing stimuli in response to determining the patient is in atrialfibrillation; sensing signals in response to the ventricular pacingstimuli; determining whether the ventricular pacing stimuli areeffectively capturing the paced ventricle over a series of the deliveredventricular pacing stimuli, the determination of whether the ventricularpacing are effectively capturing the paced ventricle comprisingdetermining a number of consecutive ones of the series that effectivelycapture the paced ventricle; and modifying a pacing rate during atrialfibrillation in response to the determination of effective capture bythe ventricular pacing stimuli delivered during the series; wherein themodification to the pacing rate is based upon the determined number ofconsecutive ones of the series that effectively captured the pacedventricle.
 14. The method of claim 13 wherein determining whether theventricular pacing stimuli are capturing the paced ventricle isperiodically performed to maximize battery life.
 15. The method of claim14 wherein periodically determining whether the ventricular pacingstimuli are effectively capturing the paced ventricle occurs solelyduring atrial fibrillation.
 16. The method of claim 13 wherein theventricular pacing stimuli are left ventricular (LV) pacing occurring ata LV tip electrode, and a monitoring vector is selected from one of aLvtip-Rvcoil or a Lvtip-device case.
 17. The method of claim 13 whereinLV the ventricular pacing stimuli are pacing is occurring at an LV ring,and a monitoring vector is selected from one of a LVring-Rvcoil or aLVring-device case.
 18. The method of claim 13 wherein the ventricularpacing stimuli are LV pacing occurring at a third LV electrode, and amonitoring vector is selected from one of the third LVelectrode-Rvcoilor third LVelectrode-device case.
 19. The method of claim 13 wherein theventricular pacing stimuli are LV pacing is occurring at a fourth LVelectrode, and a monitoring vector is selected from one of the fourthLVelectrode-Rvcoil or fourth LVelectrode-device case.
 20. The method ofclaim 13 wherein determining whether a ventricular pacing stimuli areeffectively capturing a paced ventricle occurs during normal device pacetiming operation.
 21. The method of claim 13 wherein a duty cycle modeis employed when determining whether effective ventricular capture isoccurring during AF.
 22. The method of claim 13 further comprisingdetermining whether a ventricular event is a paced ventricular event.23. The method of claim 22 further comprising acquiring an EGM inresponse to determining a ventricular event is a paced ventricularevent.
 24. The method of claim 13 wherein the ventricular pacing stimuliare delivered during one of biventricular pacing, multisite leftventricular pacing and fusion pacing.